This invention relates to a color image reading apparatus for use in a color copier and the like.
Examples of the conventional color image reading apparatus are illustrated in FIG. 4(a) and FIG. 4(b). In FIG. 4(a), the numeral 1 is an image formation lens, the numerals 3a and 3b are dichroic mirrors, and the numerals 4a, 4b, and 4c are photoelectric transfer elements which are composed of a charge coupled device and the like, wherein the photoelectric transfer elements are arranged in a line. The photoelectric transfer element is called a CCD in the remainder of this specification. A document is irradiated by a light source not illustrated in the drawing and the document image is formed on CCDs 4a, 4b, and 4c by the lens 1. The dichroic mirrors 3a, 3b are interference filters which are composed of dielectric films of low refractive index, dielectric films of high refractive index, and a transparent base board, wherein the dielectric films of low refractive index and the dielectric films of high refractive index are alternately laid on the transparent base board to form a multilayer by the method of vacuum deposition. The spectral transmittance of the dichroic mirrors 3a, 3b is shown in FIG. 6(a) and FIG. 6(b). For example, when the dichroic mirror 3a has the spectral transmittance characteristics of reflecting blue as illustrated in FIG. 6(a), and when the dichroic mirror 3b has the spectral transmittance of reflecting red as illustrated in FIG. 6(b), CCDs 4a, 4b, and 4c can read out the information of blue, green, and red, respectively. The dichroic mirrors 4a, 4b, and 4c are placed at their respective positions so that the distance between the lens 1 and CCD 4a, the distance between the lens 1 and CCD 4b, and the distance between the lens 1 and CCD 4c are equal.
In the example illustrated in FIG. 4(b), the dichroic mirrors 3a, 3b and CCDs 4a, 4b, 4c are disposed in the same way as the example illustrated in FIG. 4(a). However, prisms 81, 82, 83 are used in the example and the dichroic mirrors 3a, 3b are provided for the surfaces on which the prisms 81, 82, 83 are adhered to one another. Therefore, a ray of light enters and exits vertically with regard to the glass surface of the dichroic mirror. As a result, the occurrence of astigmatism can be prevented.
The example illustrated in FIG. 5 has a simple composition in order to obtain the information of blue (B), green (G), and red (R) components. Only one photoelectric transfer element 4 is provided, and three filters 2 are provided which can be alternately inserted in front of the lens 1, wherein the three filters 2 are defined as follows: the filter through which a blue (B) component can be transmitted; the one through which a green (G) component can be transmitted; and the one through which a red (R) component can be transmitted.
The problems to be solved can be explained as follows.
In the color image reading apparatus illustrated in FIG. 4(a) and 4(b), the composition is complicated, and furthermore incorrect color information will be obtained unless the readout image correctly corresponds to three CCDs 4a, 4b, 4c within the range of one pixel size (usually 7 .mu.m.quadrature. to 14 .mu.m.quadrature.. For that reason, the photoelectric transfer elements CCDs 4a, 4b, and 4c must be very precisely positioned. To go into more detail, an example will be described as follows. In the example, the size of a black spot on a document is equal to that of one pixel of CCDs 4a, 4b, and 4c. When the black spot image is correctly projected onto pixel N of CCDs 4a, 4b, 4c (FIG. 7(a)), the output of blue, green, and red information become zero and the color of the image is determined to be black. On the other hand, when CCDs 4a, 4b, 4c are not positioned correctly as illustrated in FIG. 7(b), signal N is determined to be green.
As explained above, when the positions of CCDs 4a, 4b, 4c are adjusted, an accuracy of the order of several .mu.m is required. Other than that, the angles of CCDs 4a, 4b, 4c to each optical axis must be adjusted. Consequently, there are problems such as many working hours needed for assembly and an increase in the cost of production.
Furthermore, there is another problem which will be described as follows. Since the discrimination between a chromatic color and an achromatic color is determined only by the information of B, G, and R, an achromatic portion made by Y, M, and C ink can not be correctly discriminated from an achromatic portion made by black ink, so that an achromatic portion such as a black letter and a gray portion can not be reproduced properly.
The composition of the color image reading apparatus illustrated in FIG. 5 is simple and the cost of production of the apparatus is low. However, the apparatus stores the data into the memory according to only the information of B, G, and R in order to discriminate an achromatic color in the same way as the color image reading apparatus illustrated in FIG. 4(a) and 4(b). For that reason, a large memory capacity is necessary to discriminate an achromatic portion and it takes time to read an image and furthermore color image reproduction is incorrect.
It is an object of the present invention to provide an improved color image reading apparatus which can solve the above-described problems, and which can correctly discriminate between an achromatic color and a chromatic color by a simple composition.